1. Field of the Invention
The present invention relates to optical elements each using a structural birefringence effect provided for minute periodic grating structures whose periods are each smaller than the wavelength of usable light, and relates to optical modulation devices, optical devices, and image display apparatuses, which are provided with the optical elements described above.
2. Description of the Related Art
Hitherto, as birefringence materials, for example, crystal materials such as calcite and quartz, liquid crystal materials, and plastic and polymeric materials having birefringence effects have been known. These birefringence materials have been used, for example, for phase plates (quarter wave plates, half wave plates) and low pass filters. In recent years, the birefringence materials have been increasingly important as materials used for various products such as liquid crystal projectors, liquid crystal displays, digital still cameras, and the like.
In addition, by forming a periodic structure having a minute period smaller than the wavelength of usable light on a substrate, a birefringence effect can be obtained. The birefringence effect obtained by the structure described above has been well known as the structural birefringence (Born & Wolf, Principles of optics 6th edition, p. 705).
As features of the structural birefringence, there may be mentioned:
(1) the amount of birefringence can be optionally controlled by design of minute periodic structures, and
(2) a large amount of birefringence can be obtained as compared to that obtained by a conventional material such as quartz.
In FIG. 39, an example of a one-dimensional grating shape having the structural birefringence effect is shown. In order to realize the structural birefringence, two materials 31 and 32 are used having refractive indices different from each other in periodic direction A of the grating. In FIG. 39, as a low refractive index material 32, air is used; however, as shown in FIG. 40, the structural birefringence can be realized in the structure in which a great number of grating materials 41 and 42 having refractive indices different from each other are alternately adhered to each other.
In the example shown in FIG. 40, the structure having a structural birefringence effect is shown in which two grating materials having different refractive indices (the grating materials 41 and 42) are used.
The structure described above can be formed, for example, by etching, electron beam drawing, LIGA process, photolithography, multiple-light-flux laser interference, or multilayer thin-film formation.
In addition, the amount of birefringence of a one-dimensional grating structure can be controlled using the refractive indices of materials, grating periods Λ, and filling factors FF as parameters. Filling factor FF is represented by a ratio (FF=w/Λ) of width w of one of two materials forming the grating shape (in the case shown in FIG. 39, the width of the material 31) to grating period Λ. For the estimation of apparent refractive indices (hereinafter referred to as “effective refractive index”) of ordinary and extraordinary light, an effective medium theory (EMT) can be used.
In a conventional technique disclosed in Japanese Unexamined Patent Application Publication No. 5-107412, a birefringence structure having a period of one half or less of light wavelength has been disclosed; however, in this publication, the birefringence structure is only described, and a phase plate having a small wavelength dependence is not described at all.
In addition, in Japanese Unexamined Patent Application Publication No. 5-333211, a phase plate in which phase differences equivalent to each other can be obtained at many wavelengths has been disclosed. However, a material forming the phase plate is composed of various anisotropic crystal plates, and a phase plate using the structural birefringence is not described.
Furthermore, in Japanese Unexamined Patent Application Publication No. 8-254607 (corresponding to U.S. Pat. No. 5,696,584), a transmissive phase grating has been disclosed; however, only a one-dimensional grating is described as the grating structure.
In addition, in Japanese Unexamined Patent Application Publication No. 9-145921 (corresponding to U.S. Pat. No. 5,847,872), a first material and a second material, having refractive indices different from each other, are used for forming a phase plate functioning as a structural refractive body; however, the structural birefringence is realized by the two materials which are alternately disposed in a plane perpendicular to the direction of usable light flux, and reduction in wavelength dependence of phase difference is not described at all.
In a recent technical paper (H. Kikuta et al. Apply Opt. Vol. 36, No. 7, pp. 1566 to 1572, 1997), a phase plate having a one-dimensional grating shape has been disclosed. However, an antireflection function is provided by forming the structure in which an antireflection film is provided on the surface of the phase plate, only the grating portion generates the phase difference, and the antireflection film itself generates no phase difference. Accordingly, in this technical paper, the phase difference between ordinary light and extraordinary light is not realized by the use of two types of materials for forming a grating portion.
Since it has been difficult to control the wavelength dependence of the amount of birefringence by using conventional birefringence materials, it also has been difficult to control the wavelength dependence of the phase difference between ordinary light and extraordinary light. When a monochromatic light source such as a laser for emitting a single wavelength is used, the phase difference can be optimized at a designed wavelength; however, in an optical system in which light including various wavelength rays, such as white light, is used, serious problems may occur when a phase plate has the wavelength dependence. For example, in the case of a liquid crystal projector using light in the visible light region, optical loss occurs at a liquid crystal panel or a color separation device, resulting in decrease in light usage efficiency of the entire system and degradation of image quality.
As described above, in order to improve the light usage efficiency and image quality in optical systems and to realize thinner optical elements, it is significantly important to use a phase plate having less wavelength dependence of phase difference in the visible light region.